Aruba, March 8, 2016 - International Women's Day has marked the plight and achievements of women for more than a century – but is now looking ahead another 15 years to the world's gender equality goals.

Themed this year around "Planet 50-50 by 2030", the United Nations-backed event will be celebrating women's rights in more than 40 countries.

It will look at how to ensure the 2030 Agenda - which positions women's empowerment as at the centre of global sustainability plans - can be concretely achieved over the coming years.

Yet the idea itself dates back more than 100 years, and has had various reasons for becoming the established celebration that it is today.

1. When was it first set up?

Socialists first put forward the idea of advancing women's suffrage through a day to mark women's enormous contribution to humankind.

An annual "international women's day" was first organised by the German socialist and theorist Clara Zetkin along with 100 delegates from 17 countries in March 1911.

The event was marked by more than one million people in Austria, Denmark, Germany and Switzerland, with hundreds of demonstrations across the Austro-Hungarian Empire.

2. Why was it set up?

When it first began, women were demanding that they be given the right to vote - which they received in Britain in 1918 but just last year in Saudi Arabia - to hold public office and to be given equal employment rights as men.

Today, when only a fifth of parliamentary seats are held by women and only 19 heads of state out of a possible 196 are women - only seven more women than 20 years ago - there is much progress still to be made.

The number of female cabinet ministers has at least tripled between 1994 and 2014 - but remains low compared to men, at only 17 per cent.

Women are also predicted to face another 118-year wait for the gender pay gap to close, with only 55 of the 500 richest people in the world being women.

3. What is this year's International Women's Day focusing on?

The United Nations first began celebrating the day on 8 March in 1975, and each year has given focus to women's status around the globe.

The current goals fit in with the 2030 Agenda for Sustainable Development.

They follow on from an event hosted by UN Women and the People's Republic of China for global leaders to commit to action on women's empowerment and access to capital.